Tuning indicator system for radio receiving apparatus and the like



G. L. BEERS 2,204,398

TUNING INDICATOR SYSTEM FOR RADIO RECEIVING APPARATUS AND THE LIKE June11, 1940.

2 Sheets-Sheet 1 Filed July 31, 1957 mmmm PM wwbm w m km PM lhwentor ear6 L. Beers attorney June 11, 1940. G. L. BEERS 2,204,398

TUNING INDICATOR SYSTEM FOR RADIO RECEIVING APPARATUS AND THE LIKE FiledJuly 31, 1957 2 Sheets-Sheet 2 I Zhwentor George L. Beers 85 I GttornegPatented June 11, "1940 UNITED STATES TUNING INDICATOR SYSTEM FOR RADIORECEIVING APPARATUS THE LIKE George LQBeers, Haddoni'ield, N.J.,,assignor"to Radio Corporation of America, a corporation of DelawareApplication July 31, 1937, Serial No. 156, 6'76 v 4 Claims.

The present invention relates to tuning or resonance indicating meansfor radio receiving apparatus and the like, and more particularly tuningindication means-which is adapted to provide resonance indicationincomparison with a condition of nonresonance, whereby receiving apparatus provided with such indication means may be tuned more accuratelythan has heretofore been possible. i

10 As is well known, in tuning radio receiving apparatus, it isdesirable to tune xactly to-reschance in order that the fidelity of'reception'may not be impaired, and, for this purpose, as a tuning aid,accurate visual means sharply responsive to a received signal is highlydesirable.

Substantially all radio receiving apparatus is provided with automaticvolume control means. As is well understood, automatic volume controldevices tend to maintain the signal applied to the second detector andtherefore the volume or audio frequency output level substantiallyconstant over an appreciable frequency range at each side of resonance,with the result that it is di-flicult to tune by car a receiver havingsucha control. A'

visual indicating device which responds to the signal amplitude at thesecond detector in such a receiver likewise cannot be used'to give asatmatic volume control that shall'give' an accurateindication ofresonance;

It is also a further object of the presentin 5 vention to provide tuningindication means for.

radio receiving systems that shall give an indication of the directionin which the tuning ele-.

- ments thereof are to be adjusted to secure res-' chance with anincoming carrier wave or signal. 40 It may also be considered to be afurther object of the invention to provide tuning indication means forradio receiving systems that shall give an indication of one type atresonance, and an indication of another and different type'at each sideof resonance. 1 I e In carrying into effect the tuning indicationoperation of the control'system in accordance with the invention, aphase discriminating net-' work, in connection withtuned coupled highfrequencycircuits, is arranged to provide a control potential which ispositive when the receiving system is tuned to one side of resonance,and negative when tunedto the other side, the magnitude of the potentialincreasing sharply as the tuningis varied from 'thej;point' ofresonance."

(Cl. 250-20) v V I It is also anobject of the present invention toprovide an accurate indication of resonance,

without requiring the use of additionalselecti've circuits even thoughthereceiving system is arranged to pass a'wide' frequency band.

The invention will, however, be better understood from the followingdescription whenconsidered in connection with the accompanying drawing,and its scope will be pointed out in the, I 10 appended claims.

In the drawings: I p s Figure 1 Ba schematic circuit diagram 'of atuning indication system embodying the invention; Figure 2 is amodification of thecircuit' of Fig. 1', arranged to provide signaldetection and amplification with automatic volume control in addition totuning indication, and 1 Figure 3 is a modification of the circuit ofFig. 1 arranged'to' utilize radio frequency impulse in' the control ofthe indicating device. 7

Referring to Fig. 1, the rectangle firepresents any suitable receivingsystem' having tuning means represented at 6. In the present example,the receiving system may be considered as including a radio frequencyamplifier, first detector and oscillator as indicated; The signalchannel' of the receiving system following the first detector comprisesan intermediate frequency amplifier tube 1 connected with anintermediate frequency input transformer 8 and having an output circuitl3 coupled to the primary winding I0 of a second intermediate frequencycoupling transformer 9. The secondary is divided into two equal windingsections II and I2, whereby it is adapted to be connected into a phasediscriminating network of the type shown, described and claimed in mycopending application, Serial No. 134,105, filed March 31, 1937, forHigh frequency signal responsive control" system, and' assigned to thesame assignee as the present application, for deriving controlpotentials through a diode rectifier device 23. i I

' As described in that application, in a control system which employs aphase discriminating network 'as 'shown, a change in either thefrequency 'of a received signal or a change in the tuning of thereceiver is'converted into a difference in phase between two voltageswhich are then applied to the phase discriminating network; By utilizingthe voltageswhich may appear across the primary and secondary of" acoupled tuned circuittransformen'l have' 'provided in one em-v bodimentof my invention, a simplearrangement for converting a change intuningfrom resonance,-

or a change in the frequency of a received signal, into a change in thephase relation between two alternating current voltages having the samefre-- quency, and means for deriving control potentials from said changein phase relation.

The primary winding is tuned to the intermediate frequency by a shuntadjustable tuning capacitor i5, and the secondary is similarly tuned bya shunt adjustable tuning capacitor Hi. The

tuned primary and secondary windings provide two coupled tuned circuitsI! and I8 resonant at the same frequency.

The windings II and I2 are arranged to form a continuous windinginductively coupled to the winding IE! and the high potential or outerterminal ends thereof are connected to the secondarytuned circuitterminals, indicated at H! and 20, and to the anode electrodes 2| and22, respectively, of a rectifier device which is included within acombined rectifier and amplifier tube indicated at 23.

The rectifier anodes 2| and 22 are associated with a common cathode 24of the device 23 which cathode is connected through a lead 25 with thetuned primary circuit ll. Continuing through the circuitv H the anodeconnection includes the positive anode supply lead 14. The secondarywinding sections are each connected with the lead l4 and a secondarycenter tap terminal 26 through two resistor sections 27 and 28. Thelatter are connected in series between the adjacent terminal ends of thesecondary sections H and i2 through leads indicated at 29. The resistorsections are provided with intermediate frequency bypass capacitors 30and 3|.

The signal or carrier potential applied between the cathode M and thecenter tap 2% of the output resistors 2'! and 28, is derived from theprimary or input circuit i'i through the connection lead 25'between thecathode, Z4 and the high potential terminal 26A. The opposite side ofthe tuned primary winding is bypassed to the negative B supply terminal52 and to ground 13 through the bypass capacitor M. Likewise the centertap 26 is bypassed to ground through the lead M and the bypass capacitor44. Direct current control potentials are derived from the resistoroutput terminals 32 and 33 through output leads indicated at 34 and 35.

In this circuit, advantage is taken of the fact that at resonance theprimary and secondary voltages of the coupled tuned circuit.intermediate frequency transformer are out of phase with each other. Byapplying the signal or carrier voltage across the primary of thetransformer between the cathode 26 and the resistor center tap 26, andthe secondary signal or carrier voltage between. the two diode anodes 2iand 22, the desired phase relations for the phase discriminating networkare obtained. If the frequency of the signal applied to the transformeris changed either because of a slight change in the tuning of thereceiver, or a change in the frequency of the transmitter, the phaserelation between the primary and secondary voltages will change. A phasechange of approximately will be obtained between the primary andsecondary voltages as the frequency of the signal applied to thetransformer is varied through the frequency range included within theresonance characteristic of the transformer.

When the signal applied to the intermediate frequency amplifier tube 7is in resonance with the transformer feeding the double diode rectifier23, a voltage will be applied between the cathode 24 and the resistorcenter tap 26 which will be 90 out of phase with the voltage developedacross the secondary coil sections H and i2 and applied between theanodes 2| and 22.

If only the voltage developed across the trans former primary is appliedto the double diode between the cathode 24 and the resistor center tap26, the rectified current flowing through the resistor section 27 willbe equal and opposite in direction to that flowing through resistor section 28 and the voltage between the terminals 32 and 33 will be Zero.Likewise if only the voltage developed across the secondary of thetransformer is applied to the double diode rectifier between the anodes2i and 22, the rectified current flowing through, the resistor section2? will be equal and opposite in direction to that flowing throughresistor section 28 and the voltage between the terminals 32 and'3t willbe zero.

If both the primary and secondary voltages are applied to the doublediode and these voltages are 90 out of phase, as will be obtained if thesignal applied to the transformer has the frequency to which thetransformer is resonant, then the signal voltages applied to the twoanodes will still be equal and the rectified currents flowing throughthe two resistor sections will be equal and opposite and the voltagebetween terminals 32 and33 will be zero.

If, however, the signal potential applied to the transformer has afrequency which difiers slightlyfrom the resonance frequency of thetransformer, the phase relation between the primary and secondaryvoltages will no longer be 90 and the signal potentials applied to thetwo anodes will not be equal and a difference in potential will existbetween the terminals 32 and 33 because the rectified currents flowingthrough resistor sections 2! and 28 will be unequal.

Assuming that the applied signal has av frequency which is just insidethe frequency response characteristic of the transformer, with aconventional intermediate frequency transformer this signal frequencywould differ from the resonance frequency of the transformer by from 2kc. to 5 kc. Under these conditions a phase difference of approximately180 would exist between the primary and secondary voltage.

Assuming that with this phase difference, the

voltages are such that the voltage applied to the anode 2i by theprimary of the transformer is in phase with the voltage applied to thisanode by the secondary of the transformer, the voltages applied to thisanode will then. add and if they are equal the anode signal potentialwill then be twice the value obtained when only the secondary voltage isapplied to the double diode. This will cause a corresponding increase inthe rectified current flowing through resistor 28. On the other hand thesignal potentials applied to the anode 22 will be 180 out of phase andif they are equal in magnitude, the sum of the voltages on this anodewill be zero. Therefore, no current will flow through resistor 2'! andconsequently no drop in potential will occur across this resistor.

Since considerable current flows through resistor 28, a relatively largevoltage will be developed across this resistor and this voltage willappear between the terminals 32 and 33; Since the current causing thisdrop in potential must flow through the circuit comprising the resistor28 and the anode 2i and cathode 24, the direction of the current will besuch that the terminal 32 will be negative with respect to terminal 33.

rag;

and increase to the initial value again. I change inpotentialyacrossterminals 32 and 33 "If the frequeny'i'ofthesignallappliedito the transformer feeding the double diode is :changedso thatit justfalls' within the otherside of the transformer frequency responsecharacteristic, the phase relation between the primary and secondaryvoltages will shift by approximately 180. Under this condition, thelarge signal potential will be applied to the. anode 22 while the signalpotential on the anode '2Iwill be substantially zero. Considerablerectified current will now flow through resistor 21 and no current willflow through resistor 28. :This current will flow through resistor 21 insuch a direction that the resultant drop in potential across theresistor causes the terminal '32 to become positive with respect to theterminal 33.

Thus, as the. frequency of the signal applied to the transformerisvaried through the frequency range corresponding .to "the transformersfrequency response characteristic, the potential across the terminals 32and 33 will decrease from a given value, pass through zero, changepolarity,

'as the frequency of the signal applied to the transformer is variedcauses a large changein control-potential to be made available for asmall change in frequency.

While the tuning indication may be "derived from any suitable portion ofthe signal channel of the receiver, the intermediate frequency amplifieris preferable for the-reason that the latter operates at a fixedfrequency and includes fixed tuned circuits at the various interstagetransformers. d

In an intermediate frequency amplifier,there-' fore, a pair of "coupledtuned circuits already existing in the signal channel of thereceiver'and operating at the same frequency may be used to derive twopotentials at the same frequency, having a phase relation which variesas the signalapplied thereto tends to; vary on either side of thepredetermined intermediate frequency.

Thus, there may be established in an intermediate frequency amplifier, aphase discriminating network which provides a differential directcurrent voltage which-varies in, magnitude and'polarity as thefrequencyof the received signal varies above or below resonance. Byconnecting a polarized indicating device or other suitableindicating'device responsive to changes in polarity and voltagemagnitude to the output leads 34 and 35,1tuning indication may readilybe obtained; which is sharply responsive to changes in tuning and whichgives a certain indication at resonance and a different type ofindication on either side of resonance. By this means, the operator of areceiver may readily determine in which direction changes in tuning areto be'made to correct for off-resonance tuning and to'bring deflectorplates ofacathode ray tube, as shown,

the cathode ray beam maybe adjusted toprovide a'figure'or"pattern' omthescreen 4| which will change with ehanges inth'emagnitude and p0- larity'of-the appl-ied c'ontrolvoltages. When resserially between the secondarywindings.

anda coupling resistor 55.

.'oriance i'sbbtained,the" figure may be a circle or spot in thecenterof the tube and, for off-resonance condition, may change to an ellipseformed on ione side or the other of the tube, thereby producing one formof indication for resonance and a differing indication foroff-resonanceJtunmary or anode circuit is returned to the center tap-ofthe resistorsx2'l and 28 and the cathode is connected therewith throughthe primary so that the rectifier circuit is at anode B potential. This.circuitarrangement permits the deflector plates to operate at positiveB potential with direct connection with the output resistors 21 and28.

If thedevice 23 in which the rectifier elements are'provided isalso adouble diode triode tube as indicated, having a control grid 45 and anoutput anode 46, the device may be arranged to provide automatic volumecontrol' bias by con- 'necting a resistor 41 between the control gridand lead I4 and connecting an automatic volume control lead 49 to thegrid end of resistor 41, as

shown. With this arrangement, the primary voltage is rectified by thediode rectifier action of the tube in which the ugridacts as an anode ofthe rectifier. A suitable intermediate frequency bypass capacitor 50a isprovided across the automatic volume control resistor 41. Signal outputpotentials may-then be derived from -the secondary circuit between theterminal 26 and the terminal 32. Audio frequencyor modulation potentialsmay: be derived from the resistor 41. The value of the capacitor 5011should be such that it provides an effective jb-ypass at intermediatefrequencies 5 without appreciably attenuating the audio frequencypotentials in the desired'range of operation.

A circuit for deriving signal potentials and automatic volume "controlpotentials is also shown in Fig. 2, to which attention is now directed.In Fig. 2, like parts as in Fig. 1 are designated by the same referencenumerals.

In the present example, the doublediode rectifier is provided by a tube50 which may be of the type known as an RCA 6H6 having two anodeelectrodes 5| and 52 each associated with a cathode 53. The anodes 5|and 52 are connected with the terminals I9 and 20 ofthe tunedsecondarycircuit 18 which comprises the secondary winding sections I l and I2 andthe shunt tuning capacitor Hi. The resistor sections 21 and 28 areincluded Output leads 34 and 35 are connected with output terminals32and 33 on the resistors and the lat: ten are provided with bypasscapacitors 3B and 3|. 4

-'Thecenter tap ZG on the resistor'elements is connected with thecathode 53 through a lead 54 p The output anode circuit 13 of theamplifier I is connected withthe 'primarywinding Ill which is providedwitha shunt variable tuning-capacitor 15 to form the tuned primarycircuit I1, and the latter is coupled to thecathode 53 through acoupling capacitor '55 in conjunction with the coupling resistor 55 vThe operation of the circuit of Fig; 2 is similar to that of Fig. 1 andthe tuning indicator device is connected with the output resistorcomp-rising the sections 2'! and 28 in a similar manner to the cathoderay tube of Fig. 1.

In the present example, the tuning indicator device is an electricalmeter indicated at H having a movable pointer 12 which moves over anindicating scale is on either side of a zero position in which it isshown at 14.

The meter is connected to the leads 34 and 35 to receive thedifferential potential from the output resistor 21-23 and the indicatormoves to the rightor left as the potential increases from zero in eitherdirection depending upon the polarity of the terminals 32 and 33, onewith respect to the other.

The derivation of the variable potential is the same as in the circuitof Fig. 1 except that the cathodes 53 are conductively connected back tothe center tap 26 through the primary circuit by way of the couplingresistor 55 with which the primary is coupled in shunt relation throughthe coupling capacitor 56, thereby permitting grounding of the secondarycircuit. A choke coil may be substituted for the risistor 55 if desired.

{my suitable polarized indicating device may be used in connection withthe leads 3t and 35 to respond to the signal variable potential.

The circuit has the advantage that the modulation or audio frequencycomponent of the received signals may be applied to an outputampliflying circuit through a second detector comprising a dioderectifier 59 connected with onehalf of the secondary winding. In thepresent example, a cathode 6i! is connected with the terminal 25 of thetuned circuit l8, and the anode electrode 6| is connected through anoutput resistor 62 for the detector, to a ground lead 63 connected tothe terminal 32. The output resistor is provided with a suitable by-passcapacitor 64.

The intermediate frequency carrier wave received in the tuned circuit I3is applied in part to the rectifier 58 across one-half of the secondarywinding l2, and the modulation and direct current component thereofappear across the output resistor 62.

The output resistor 62 may be connected between the control grid 65 andthe cathode 66 of an audio frequency amplifier 61 to apply a biasingpotential thereto and audio frequency signals simultaneously, wherebythe amplifier is diode biased by the signal. The audio frequency outputis derivedfrom an output lead 68 in connection with the output anode 69.The cathode 66 is connected with the ground lead 63, as indi. cated.Automatic volume control potentials may be derived from the outputresistor 62 through an output lead indicated at H1 and applied to anysuitable portion of the receiving system for automatic volume controlpurposes, in any wellknown manner.

In the embodiment of my invention illustrated in Fig. 3, the radiofrequency potentials developed across the primary and secondary oftransformer 9 are applied directly .to the deflecting plates 35 and d!)of the cathode ray tube 3'! and the rectifier tubes used with thearrangements shown in Figs. 1 and 2 are eliminated. By applying thevoltage across the primary of transformer 9 to the vertical deflectingplates and the voltage across the secondary to the horizontal defleetingplates a circle will be formed on the screen of the cathode ray tubewhen the applied signal has the frequency to which the trans former'isresonant and provided that the voltages applied to the two sets ofdeflecting plates are substantially equal. If the frequency of theapplied signal is varied on either side of the frequency to which thetransformer is tuned the figure on the screen of the cathode ray tubewill change from a circle to an ellipse the inclination of which willindicate whether the frequency of the applied signal is above or belowthe frequency to which the transformer is tuned. In this embodimentresistors 21 and 28 and by-pass condensers'3ll and 3| can be eliminatedso that a single coil can be substituted for windings II and I2.

While the tuning indication system has been shown and described inconnection with an intermediate frequency amplifier, it may be appliedto other portions of the radio receiving system wherein two circuits areprovided which operate at the same signal frequency and which providesignal voltages which vary in phase in response to off-resonance tuning.However, for reasons which have hereinbefore been pointed out, thetuning indication system is at present preferred in connection with theintermediate frequency amplifier, since this connection permits existingapparatus to be utilized to advantage.

I claim as my invention:

1. In a radio signal receiving system, the combination of means fortuning said system to resonance with a received signal, means providingtwo substantially equal signal potentials in response to a receivedsignal, said potentials having the same frequency and a predeterminedphase relation to each other when said receiving system is adjusted toresonance with said received signal, means for converting a variation infrequency of said received signal into a variation in phase between saidpotentials, said first and second-named means including the primary andsecondary of a coupled tuned circuit intermediate frequency transformerthrough which said received signal is conveyed, means for convertingsaid variation in phase into a controlling potential for said system,said controlling potential being variable in response to said phasevariation in magnitude and direction of variation from a zero value,indicating means responsive to changes in the magnitude and direction ofvarir ation of said potential, whereby resonance and off-resonanceconditions of tuning of said system are provided thereby, a signalrectifier connected with said first-named means for rectifying at leasta portion of one of said signal potentials, and means for deriving themodulation component of said signal and automatic volume controlpotentials from said last-named rectifying means.

2. In a superheterodyne receiver, the combination with tuning meanstherefor, of an intermediate frequency amplifier, a coupling transformerfor said amplifier having a tuned primary circuit and a tuned secondarycircuit resonant at the same intermediate frequency, said secondarywinding having two tuning inductance sections, a resistor element havinga center tap connected between said sections, a rectifier device havinga cathode and two anode electrodes associated therewith, said anodeelectrodes being connected with the terminals of said secondary circuitand said cathode being connected through the primary circuit with saidcenter tap on the resistor element, means for deriving difierentialdirect current potential from said resistor element in response toreceived signals, tuning indication means responsive to the magnitudeand polarity of said potential connected with said resistor element,'andmans-for deriving audio frequency signal and automatic volumecontrolpotentials from at least one portion of said secondary circuit.

3. In a radio receiving system, the combination with means fortuningsaid system to resonance with a signal, of a signal amplifierprovidedwith two tuned coupled signal conveying circuits, said circuits beingresonant at substantially the same signal frequency, said tuned circuitsincluding the primary and secondary windings of a tuned transformerdevice, a rectifier having a cathode connected through the primarywinding with acenter tap on the secondary winding and having a pair ofrectifier anode electrodes connected with the terminals of the secondarywindings, an output resistor for said rectifier, a cathode rayindicating device having a pair of beam deflecting electrodes connectedwith said output resistor, an

audio frequency detector connected with one of said windings, and meansfor deriving audio fre-f quencyand automatic volume control potentialsfrom said last named detector.

4. In a superheterodyne receiving system, the combination with anintermediate frequency amplifier, of an in'terstage coupling transformertherefor having a tuned primary and a tuned secondary winding responsiveto the same intermediate' frequency, apair of resistors interposed inthe secondary providing a center tap therefor, a rectifier having acathode connected through said primary winding with said center tap andhaving a pair of anodes connectedwith the terminals of the secondary, acathode ray indicating tube having at least one pair of beam deflectingelectrodes connected with one of said windings to receive a differentialcontrolling potential "therefrom responsive to a variation in frequencyof a signal,

a rectifier device connected with a portion of the secondary winding andhaving an output resistor, and an audio frequency amplifier connectedwith said resistorto receive output signals therefrom GEORGE L. BEERS.

